The special properties of a shape memory alloy (SMA), such an alloy of nickel and titanium, known as nitinol, have enabled the creation of a new generation of medical devices. Unlike traditional metals or alloys such as stainless steel, titanium and tantalum, such alloys exhibit shape memory properties through a solid to solid phase change which occurs with a change of temperature over a certain range. A plastically deformed device formed of a SMA when heated over a certain temperature range will “remember” its original shape and return to it.
The shape recovery of a plastically deformed device formed of a SMA upon heating is called “one-way shape memory effect.” The more important parameters of a one-way shape memory effect include recovery strain (εr), shape recovery rate, recovery stress (σr) and the temperatures at which direct martensitic and austenitic, i.e. reverse martensitic, transformations occur, designated Ms, Mf, As and Af.
As used herein, in the case of Ni—Ti alloy, enriched with nickel and aged, there is a two-stage transformation upon heating or cooling. Ms designates the martensite start temperature which is the temperature at which the transformation from the intermediate R-phase to martensite begins on cooling; Mf designates the martensite finish temperature which is the temperature at which the transformation from R-phase to martensite is completed upon cooling; As designates the austenite start temperature which is the temperature at which the transformation from the R-phase to austenite begins on heating; and Af designates the austenite finish temperature which is the temperature at which the R-phase to austenite transformation is completed on heating.
A two-way shape memory effect (TWSME) can be induced in a device formed of a SMA by a single or repeated deformations of the device in conjunction with thermocycling through a temperature range over which martensitic and austenitic transformations occur (“a so-called training”). Devices formed of a SMA having an induced TWSME “remember” both their low temperature shape and high temperature shape, so that appropriate cooling of a device formed of a SMA in which a two-way memory has been induced will cause the device to return towards its deformed shape.
A TWSME of a SMA is characterized by its own TWSME parameters including two-way recovery strain (εTW), shape recovery rate, recovery stresses (σr) and temperature ranges over which a shape recovery occurs. The value of these parameters can be adjusted by subjecting a device formed of a SMA to certain thermomechanical and heating treatments. Moreover, the stability of the two-way recovery strain and the temperature range over which a TWSME occurs is of great practical importance.
Several circumstances must be taken into account when training a device formed of a SMA to have TWSME, especially in medical applications such as endovascular stents:
1. The two-way recovery strain (εTW) must be at a sufficiently high level to provide the required functionality of the device.
2. The temperature ranges (cooling and heating) over which the two-way shape change is realized must be appropriate for the application (for example, in endovascular stents situated in a blood vessel, heating to a temperature not higher than the temperature of blood coagulation, i.e., about 42° C., and cooling to a temperature not below about 10° C.).
3. The requirements of items 1. and 2. become even more important when reiterative shape changing upon heating and cooling is necessary. Under such circumstances, the stability of the two-way recovery strain (εTW) and the temperature ranges over which the two-way shape changes occur become very important.
4. The TWSME parameters of the SMA should be substantially constant during thermocycling through the temperature ranges over which the two-way shape changes occur, as well as during subsequent storage of devices made from the SMA.
5. Even after accidental heating of the SMA device over the final temperature of shape recovery Af, the two-way recovery strain (εTW) and the temperature ranges over which the two-way shape changes occur must remain stable.
Any reduction in the two-way recovery strain must be minimal, and the two-way shape changes must still occur within the required temperature ranges.
At the same time, the properties of Ni—Ti alloys must be taken into account:
1. The shape change and shape recovery of medical devices, such as stents, formed of a SMA having an induced TWSME are realized by austenite (B2)→intermediate R-phase (R)→martensite (B19′) and B2→R→B19′ phase changes, respectively. A TWSME is realized by setting up permanently acting oriented microstress fields by strain hardening (under external stress) or by transformation induced hardening under B2RB19′ phase transformations or by oriented coherent precipitates.
2. Continued phase transformation induced hardening and strain hardening have their own influences upon temperatures of B2R and RB19′ transformations. According to different sources this influence is ambiguous. These changes are at their maximum in the initial phase transformation cycles (or in the initial cycles of active deformation), and gradually degrade with subsequent cycles. As a result, the difference between TR and MS in the initial cycle can reach 60°-80° C. after ageing. The foregoing factors should be taken into account in connection with TWSME training of a device formed of a SMA by providing that the As and Af temperatures of the SMA device should be significantly lower than their respective values for the finished TWSME trained device.
3. The influence of external action parameters of TWSME training, e.g. strain and loading modes, loading temperature regime or phase state, load value, loading time and number of training cycles, on the magnitude of the εTW should be taken into account. It is well-known that external action parameters of TWSME strongly affect kinetics and final TWSME parameters.
Various techniques for inducing a TWSME in a SMA have been suggested. For example, see U.S. Pat. Nos. 5,624,508, 5,836,066, 5,882,444 and 6,596,192. The '508 and '444 patents disclose subjecting a SMA to various combinations of heat and thermomechanical treatments (including ageing, polygonizing annealing and recrystallizing annealing) to induce a TWSME. These techniques have not proven to be satisfactory.
For example, known methods of TWSME training do not achieve sufficiently high values of εTW in the preferred temperature range of 10° C.-37° C. in which shape changes of medical devices such as stents formed of SMA having TWSME should occur.
Known TWSME training techniques are based on martensitic transformations and experience has shown that thermocycling through a full temperature range of martensitic transformation(s) results in decreases of transformation critical temperatures which in turn downwardly shift the temperature range over which the shape-changing TWSME transformations occur.
Moreover, known TWSME training methods do not provide thermal stability of the TWSME parameters induced through R-phase. The two-way memory effect degrades when the SMA is heated above Af and is not fully restored with repeated thermal cycling.